1. Field of the Invention
The present invention relates to an optical scanning apparatus, suited to high print dot density printing, for performing parallel scan by simultaneously using a large number of laser beams that can be modulated independent of one another.
2. Description of the Related Art
It has been well known that a simultaneous and parallel scanning system using a plurality of beams is effective as means for managing both the increasing of a printing speed and the enhancement of a print dot density in a laser printer adapted to write image information by basically utilizing laser beam scanning. Such a conventional scanning system is, for example, (1) a two-beam scanning system that uses two laser light sources adapted to output a single output light beam and that is enabled to control the interval between the two beams in a scanning surface after the two beams are collectively deflected and caused to scan (see Japanese Patent Application No. 60-86446), (2) a system adapted to split a single beam outputted from a laser light source into a plurality of beams and to use the plurality of beams as scanning beams after each of the beams is passed through a corresponding light modulator and then deflected and used for scanning (see JP-A-53-146644), (3) a system adapted to connect output beams of individual semiconductor lasers to one another by an optical director element and to constitute an array light source by placing output end portions of this optical director element in such a way as to be close to one another (see JP-A-54-7328), (4) a system using an array type semiconductor laser constituted by incorporating a plurality of laser devices, which can be driven independent of one another, into a single semiconductor laser light source (see Japanese Patent Application No. 53-66770), (5) a system that uses two two-element array light sources and has a mechanism for adjusting the positions of the light sources in such a way as to set the interval between adjacent scanning beams at a preset value (see JP-A-3-107910), and (6) an optical system adapted to detect and adjust a beam pitch interval for performing two-beam scanning (see JP-A-9-193465).
When image information is recorded by using a multibeam scanning optical system, inconsistency in density occurs in the case that the beam interval in a direction perpendicular to the scanning direction shifts from a proper value according to a print dot density. This results in degradation in picture quality of a printed image. Thus, the prior art system (1) performs a servo control method at each beam scan to thereby stabilize the beam interval. It is, however, difficult to apply this prior art system to multibeam scanning in the case that the number of simultaneously used scanning beams is 3 or more. The prior art systems (2) to (4) do not take the stabilization of the interval between adjacent ones of scanning lines into consideration. The prior art system (5) is adapted to detect individual scanning line positions in the case of two-beam scanning in the interval between print jobs and adjust the interval between the scanning lines. This prior art system, however, is unsuitable for the cases that a long-duration continuous operation is needed, and that the number of simultaneously used scanning beams is 3 or more.
In particular, in the above conventional devices (3) and (4) (see JP-A-54-7328 and Japanese Patent Application No. 53-66770), when image information is written by using light sources of such conventional systems and performing parallel scanning thereon with a plurality of beams, inconsistency in density occurs in the case that the value of the beam interval between scanning beams corresponding to a predetermined print dot density is not a prescribed value according to a print dot density. This results in degradation in picture quality of a printed image. A cause of an occurrence of variation in the beam interval is that the position of the array light source portion shifts from a predetermined stationary position owing to change in ambient temperature and to mechanical shock, and that this shift causes variation in angle of arrangement of light source light emitting portions with respect to a main scanning direction of beams.
A countermeasure against this variation in beam interval is a system, in which a CCD line sensor is placed near to a scanning end of the scanning recording medium to thereby detect the position of each of the scanning beams, and in which a beam pitch is adjusted to a set value of the beam interval by rotating the array light source when the beam pitch obtained from the detected positions differs from a currently selected set value of the beam interval as proposed by the above conventional device (6) (see JP-A-9-193465). In this system, the position of a single scanning beam is detected by performing scanning once. Upon completion of measurement of the beams, the beam pitch is detected. Then, the position of the light source is corrected. Further, the detection and correction of this beam pitch are performed before the commencement of a printing job, or during the interval between jobs. According to this system, the pitch intervals between the scanning beams cannot be detected simultaneously with the scanning by using the beams. Thus, the accuracy, with which the beam pitch or interval is detected, is degraded. Moreover, the detection and correction of the beam pitch cannot be performed during operations of a printer. Therefore, this conventional system is unsuitable for a job, the execution of which requires a long time.
The invention has been made to solve the aforementioned problems, and therefore one object of the present invention is to provide an optical scanning apparatus adapted to detect the interval between scanning lines respectively corresponding to a plurality of beams at each beam scanning without interrupting a printing job, and also adapted to perform a stabilization control operation on the interval between scanning lines so that the interval therebetween stably has a set value, and enabled to be applied to the case that a large quantity of image information is recorded and processed at a high speed and at high resolution.
To solve the foregoing problems, a conventional array type semiconductor laser device shown in FIG. 2 is used as a light source therefor. When this light source is, for instance, an array light source having three laser devices, a plurality of laser devices 91, 92, and 93, which can be modulated independent of one another, are formed on the same substrate 90. The light emitting portions 95, 96, and 97 are arranged at equal intervals in a straight line. Although this example is described herein, for simplicity of description, in the case that the number of laser devices is 3, the following holds regardless of the number of laser devices. Output beams 11, 12, and 13 (or 21, 22, and 23) of the laser devices have nearly equal wavelength and intensity. Moreover, the polarizing directions 114, 115, and 116 (or 214, 215, and 216) thereof are the same as an arrangement direction 119 in which the laser devices are arranged. Two such semiconductor laser light sources are used. Output beams 11, 12, and 13 (21, 22, and 23) of two such semiconductor laser light sources are combined into a bundle of composite beams. The surface of a photosensitive drum, which is an image recording medium, is scanned with the composite beams, which are collectively deflected and pass through a rotary polygon mirror and an FE) lens. Therefore, image information is written to the medium by performing parallel scanning thereof by simultaneously using beams, the number of which is the sum total of beams outputted from each of the laser light sources. The beams are separated on the surface of this drum from one another at intervals determined according to a print dot density. The beams are modulated in intensity independent of one another according to image signals.
Incidentally, it is necessary for realizing high picture quality of a printed image obtained by multibeam scanning that the intervals of the scanning positions of the beams are always at a set value. Thus, the stabilization of the interval between adjacent ones of the beams is achieved by providing a device for detecting the interval therebetween at a place in the vicinity of a scanning start edge and for correcting a shift from a set value when such a shift occurs. Furthermore, a countermeasure against a scanning beam position shift caused owing to a shift in relative position between the array light sources 1 and 2 is to draw out a part of light beams from each of the light sources and to stabilize the interval between the scanning positions of these beams at the set value.
The aforementioned means according to the present invention employs two array light sources each having a practically available number of devices mounted therein. Thus, as compared with the prior art apparatus having a single array type semiconductor laser, the number of scanning beams is doubled in the apparatus according to the present invention. Consequently, the processing speed is increased still more. Furthermore, high dpi setting is achieved.
Another object of the invention is to realize a multibeam scanning optical system enabled to detect the pitch or interval of scanning beams simultaneously with detection of each of the beams, and to always detect and correct the pitch of the scanning beams even during operations of a printer, and to maintain the pitch or interval of the scanning beams at a set value preferably for a high-speed laser printer that performs a long-duration continuous operation.
In a laser scanning optical system applied to a laser printer, usually, a photodetector for determining the timing, with which a scanning beam passes through a predetermined position, is placed near to the scanning start edge of a recording medium, such as a photosensitive drum. Meanwhile, output portions constituting an array light source are arranged at equal intervals. Further, the array light source is disposed in such a manner as to be inclined to the scanning direction so as to set the scanning pitch or interval on the scanning surface, which is a recording medium, at a value corresponding to the print dot density. At that time, all the time intervals between the moments at which the successive beams pass through the photodetector, respectively, are equal to one another and to a value that changes according to the inclination angle of the array light source. When the angle or direction of arrangement of the array light source changes, the interval between the scanning lines on the scanning surface varies. This results in variation in time interval between moments at which the successive beams pass through the photodetector, respectively. The interval between the scanning beams can be maintained at a constant value by detecting this variation in the time interval, and adjusting the direction of arrangement of the array light source according to this detection signal to thereby correct and control the time interval at a proper value. This is indispensable for realizing high picture quality of a printed image.
Further, the detection and correction can be performed at each scanning according to the invention, without interrupting a printing job. Moreover, even during an operation of a printer, the checking and correction can be always and continuously performed.